1. Technical Field
The present invention relates to a gating grid for deflecting ions and a method of making same.
2. Description of the Related Art
Diverting the path of high-speed charged particles is important in many areas of applied physics. Scanning Electron Microscopes (SEMs) use deflection plates parallel to an electron beam to steer electrons across a surface to develop an image. Similarly ion implanters used in the manufacture of microchips also employ charged plates to steer dopant atoms to desired locations on a chip. In both of these applications capacitively charged plates apply electromagnetic normal forces to high-speed charged particles to divert or steer the path of the particles.
Bradbury-Nielson (B-N) gates were first invented in 1936 and were used as electron filters. Similar to the charged plates described above, a Bradbury-Nielson gate is a gating grid that steers charged particles through the use of an electromagnetic field. However instead of employing charged plates, a B-N gate uses two interdigitated combs of wires that are electrically insulated from each other. If alternate potentials are applied to the two wire combs, a charged particle passing between two of the interdigitated wires will be pushed away from one wire and pulled toward the other, steering the particle from its initial path.
More recently B-N gates have been used to deflect ion beams in time-of-flight mass spectrometry (TOF-MS). Certain applications of TOF-MS require identifying particular packets of ions as they move through a drift tube toward a detector. If a B-N gate is placed inside such a drift tube and the charged wires are modulated between different potentials at a high frequency, desired ion packets can be selected for analysis and undesired ion packets can be discarded. For example the packets that pass through the gate when the gate is on may be diverted away from the detector, and only the ion packets that pass through the gate when the gate is off will reach the detector.
Such modulation of B-N gates is more efficient than modulating charged plates. The electromagnetic field surrounding the thin wires of a B-N gate can be made much smaller than the field surrounding two charged parallel plates. Therefore ions passing through a B-N gate spend less time exposed to its electromagnetic field and a square wave “pulse” used to modulate the field can have much sharper edges and a higher frequency. The sharp edges and high frequency (often greater than 10 MHz) of such a pulse in turn mean that the B-N gate can be more discriminating and select smaller packets of ions.
Similarly, if the spacing between the wires of a B-N gate is decreased, the required potential across the wires of the B-N gate that is needed to divert an ion packet is reduced and the modulation frequency of the gate can be further increased.
Therefore considerable efforts have been expended to minimize the spacing between wires of B-N gates. Prior art B-N gate manufacturing techniques however have involved hand manipulations of the fine wire combs. Even with advanced microscopic tools, hand manipulations of such tiny features is extremely difficult, time consuming and costly. U.S. Pat. No. 5,465,480 to Karl et al. discloses a method of manufacturing a B-N gate where the parallel fingers of the gate are cut or etched into a metal foil. The resulting metal grid is then stretched and glued to an insulating ceramic frame. Finally, the connections between the grid elements are individually severed to create the interdigitated fingers of the B-N gate.
In 2000 Brock et al. reported producing a single B-N gate with wire spacing of 0.16 mm by stretching gold-plated tungsten wires across a frame and gluing each wire in place using a polymethylacrylamide spacer. A. Brock, N. Rodriguez, and R. N. Zare, “Characterization of a Hadamard Transform Time-Of-Flight Mass Spectrometer,” Review of Scientific Instruments, Vol. 71, No. 3, 1306 (2000).
In 2001 Kimmel et al. disclosed a semi-automated method of producing B-N Gates to produce spacing as small as 0.075 mm between wires and that requires around three hours to make each gate. Kimmel et al. employs a hand-cranked tool that stretches a continuous wire along a grooved polymer block. J. R. Kimmel, F. Engelke, and R. N. Zare, “Novel Method for the Production of Finely Spaced Bradbury-Nielson Gates,” Review of Scientific Instruments, Vol. 72, No. 12, 4354 (2001).
Another technique for reducing the potentials required to divert ions using a B-N gate is described in U.S. Pat. No. 5,986,258 to Park. Instead of decreasing the spacing between the wires of the B-N gate, the '258 patent discloses a type of three-dimensional B-N gate employing stacked parallel deflection plates. This technique is a compromise that decreases the relatively high voltages of traditional B-N gate wires but at the cost of diminished resolution due to an extended electromagnetic field.
There is therefore a need for a more automated method of manufacturing gating grids with decreased spacing between the wires, that requires less manufacturing time per gate and that is less costly.